1. Field of the Invention
An embodiment relates to a method and a system for handover of a terminal in a radio communication system with hierarchy cell architecture in which macrocells and femtocells are overlapped on each other. More specifically, an embodiment relates to a method and a system for adaptively controlling handover between macrocell base stations, between femtocell base stations, or between femtocell base station and macrocell base station, when a serving base station has load increased or decreased by a predetermined degree.
2. Description of the Related Art
A radio communication system uses electromagnetic waves for communication between stationed and mobile radio communication devices such as wireless mobile phone located within a communication coverage area or cell, or laptop computer having radio communication card therein. The base stations are deployed spatially in a geographical service area which is divided into radio cells, to provide radio coverage. In operation, the base stations send out information to a mobile terminal through a down-link radio signal generated therefrom. A terminal located at a predetermined cell sends out the information to a serving base station of the specific cell through up-link radio signal. The base station may have oriented antenna to further divide each cell into different cells or sectors, and each antenna covers one sector. This sectoring of cell increases communication capacity. A variety of radio communication systems, such as a portable device, a mobile phone, a wireless card, a mobile terminal (MT), a user equipment (UE), an access terminal (AT) or a subscriber station (SS), may include one or more base station network to communicate with one or more wireless devices. The base station may be called an access point (AP) or an access network (AN), or included as part of a network. Further, the radio communication system may include one or more access networks to control one or more base stations.
In a radio communication network, a base station may be implemented in multi-tier architecture. By way of example, a base station may be located in a radio cell of another base station to provide the radio coverage of a small part of a radio cell. In this case, because another base station is located in a macrocell area, a larger cell can be considered as macrocell, while the smaller cell within the macrocell can be considered as microcell. This macrocell-microcell architecture may extend the radio coverage of a network, and increase radio frequency band and communication capacity of the network. One macrocell may include one or more microcells depending on need for radio coverage at the macrocell.
Efforts have been made to increase cell capacity and thus support bidirectional service with high capacity service such as multimedia content, streaming, etc., and along with this, approaches to use high frequency band and decrease cell radius have also been made. Using cells such as picocell with narrow cell radius enables use of higher band than the frequency used in conventional radio communication system, and therefore, more information transmission is provided. However, cost increases because more base stations have to be installed in the same area. The femtocell has been recently suggested as one of the approaches to increase cell capacity using small cells.
The femtocell refers to a small scale radio environment in which ultra-small base stations consuming low power are installed inside the home/office building. The femtocell is expected to contribute complete settlement of the next-generation mobile communication system, by providing improved service quality such as improved indoor service coverage and increased capacity.
FIG. 1 is a view illustrating a conventional radio communication system, in which macrocells provided by a macro base station and femtocells provided by a femto base station are overlapped with each other.
Referring to FIG. 1, the femto base station generally has smaller cell coverage than that of macro base station, and whole (not illustrated) or part of the cell coverage of the femto base station can be included in the cell coverage of the macro base station.
FIG. 1 illustrates a situation in which a first terminal 301 and a second terminal 302 receive service from a first macro base station 110 applied as a serving base station. The terminals 301, 302 are mobile and the first terminal 301 is at a location where the cell coverage of the first macro base station 110 overlaps the cell coverage of the adjacent femto base station 200, and the second terminal 302 is at a location where the cell coverage of the first macro base station 120 overlaps the cell coverage of the second macro base station 120.
As explained above, the terminals 301, 302 at locations of overlapping cell coverage between the respective base stations frequently determine whether to hand over to another base station. The first macro base station 110 (i.e., serving base station) or the terminals 301, 302 compare to determine if the strength of the signal (RSRPM_i or RSRPF_i) received from the adjacent base station (i.e., second macro base station 120) or the femto base station 200 to the terminals 301, 302 exceeds the sum of the strength of the signal (RSRPS_k) received from the serving base station (i.e., first macro base station 110) and the handover threshold, to thereby determine if it is necessary to hand over to the second macro base station 120 or to the femto base station 200. That is, if the strength of the signal (RSRPFj) received from the femto base station 200 is equal to or greater than the strength of the signal (RSRPS_k) received from the femto base station 200 by the handover threshold (HOMF), the first terminal 301 hands over from the first macro base station 110 to the femto base station 200.
In the same situation, the second terminal 302 is at an area where the cell coverage of the first and second macro base stations 110, 120 overlap. In some situations, the strength of the signal (RSRPMj) received from the second macro base station 120 may be greater than the strength of the signal (RSRPS_k) received from the first macro base station 110, but with the difference that does not exceed the handover threshold (HOMM). In these situations, according to the above method for determining whether or not to hand over, the second terminal 302 does not hand over to the second macro base station 120, but keeps being served by the first macro base station 110.
If the number of terminals in the same situation as the second terminal 302 increases, i.e., if the number of terminals which do not hand over to another base station increases, the traffic load of the serving base station 110 may increase to overload state. If this happens, terminals served by the serving base station can have interference therebetween, and the above-explained handover determining method can hardly solve the traffic overload, while the terminals have deteriorated QoS and transmission rates. Accordingly, a method and a system are necessary, according to which terminals 301, 302 can easily hand over to adjacent base stations 120 or 200 when the serving base station is overloaded.